Explain why sounds travelling through air are difficult to transmit into the fluid-filled cochlea, and describe how the ear overcomes this problem.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Sound waves in air travel through a low‑density medium and must enter the denser fluid of the cochlea. At the air–fluid interface most of the acoustic energy is reflected because of the impedance mismatch. The middle ear solves this by the ossicular chain (hammer, anvil, stirrup) which transmits and amplifies the vibration from the eardrum to the oval window. The oval window has a much smaller area than the eardrum, so the same force is applied over a smaller area, increasing the pressure. This higher pressure allows sufficient energy to be transmitted into the cochlear fluid, overcoming the impedance mismatch.
Examiner tips
- Use the term ‘impedance mismatch’ to show understanding of the physics.
- Explain the role of the ossicular chain and the area difference between eardrum and oval window.
- Show how increased pressure transfers energy into the fluid.
- Keep the answer concise – 5 marks only.
Common mistakes
- Confusing density with impedance; students say ‘density’ instead of ‘impedance mismatch’.
- Forgetting to mention the area difference between eardrum and oval window.
Mark scheme (5 marks)
- Sound travels through air (a low-density / less dense medium) and must enter fluid (a denser / more dense medium)
- Most sound energy is reflected / lost at the air–fluid boundary without the middle ear
- The ossicles (hammer / anvil / stirrup) transmit and amplify vibrations from the eardrum to the oval window
- The oval window has a much smaller area than the eardrum, concentrating force / increasing pressure
- The increased pressure allows sufficient energy to be transferred into the cochlea fluid / overcomes the impedance mismatch
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Physics of the Ear questions
- Explain why a person with a damaged ossicle chain may struggle to hear quiet sou…
- Describe how the loudness and frequency of a sound are represented in the vibrat…
- Explain why sounds above 20 000 Hz are not heard by most adults, and why older a…
- Explain how the structures of the ear convert sound waves in air into electrical…